1. Field of the Invention
The present invention relates to servo control devices for use in testing tension specimens where crack growth is to be analyzed.
2. Prior Art
Testing of tension specimens to determine crack growth rates has long been carried out. It has been of interest in testing to determine the length of the crack in relationship to load applied and to the number of cycles of operation, and from this information analyze the properties of material from which the specimen is made.
The copending United States Patent Application of William D. Juusola for Test Specimen Crack Correlator, Ser. No. 603,135, filed on even date herewith, discloses means for directly obtaining electrical signals proportional to the crack length of a test specimen. In doing so, a polynomial equation comprising a third order equation that describes the curve of a compliance factor of the specimen plotted versus a quantity comprising the crack length divided by the specimen width is electrically solved.
It has also been recognized in the test field that the stress intensity factor, which describes the severity of stress at the end of the crack in a specimen is a parameter that is of interest in predicting crack growth rates, and otherwise analyzing the effects of loading. The stress intensity factor is used as an independent parameter in formulating fatigue crack growth studies. Previous studies of the influence of stress intensity factor have relied on values that are calculated from load and crack length data after the test. Thus, while recognized as the significant parameter, the stress intensity factor has not heretofore been a parameter that could be usefully used in direct control of tests, or in direct analysis of crack growth.